A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, such as a mask, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
To reduce the size of features that can be imaged using a lithographic apparatus, it is desirable to reduce the wavelength of the illumination radiation. Ultraviolet wavelengths of less than 180 nm are therefore currently used, for example, 157 nm or 126 nm. Also used are extreme ultraviolet (EUV) wavelengths of less than 50 nm, for example, 13.5 nm.
While gaseous contamination particles in lithographic apparatus continue to be a problem, it has been found that apparatus operating at shorter wavelengths, such as those less than 180 nm, are significantly more sensitive to the presence of contaminant particles than those operating at longer wavelengths. Contaminant particles such as hydrocarbon molecules and water vapor may be introduced into the apparatus from external sources, for example, gas supplies, wafers, or masks, or they may be generated within the lithographic apparatus itself, for example, by degassing of equipment.
It has been found that contaminant particles tend to adsorb to the optical components in the apparatus and cause a loss in transmission of the radiation beam. When using 157 nm or EUV radiation, a loss in transmission of about 1% is observed when only one or a few monolayers of contaminant particles form on the optical surfaces. Such a loss in transmission is unacceptably high. Further, the uniformity requirement of the projection beam intensity for such apparatus is less than 0.2%. Contamination compromises this requirement.
There is also a risk that the adsorption of contaminant particles on the surface of the optical components, or within the optical surface in the case of a porous surface, for example, an anti reflection coating, may cause damage to these optical components. Damage may occur if the optical components are suddenly irradiated with UV radiation, for example, 157 nm at a high power. The irradiation may cause rapid evaporation of the smaller contaminant particles, such as water molecules, which may be trapped within the porous surface of an optical component, thereby causing damage to the optical component itself. Such damage requires costly repair or replacement of components. Further, adsorption of contaminants to optical components such as reflectors and lenses (in lithographic apparatus operating a longer wavelengths) has been found to reduce the operating lifetime of such components. Further, cracking of molecules reduces the operating lifetime of components. For example, during EUV exposure, hydrocarbons are cracked to generate carbon which leads to build up of carbon layers on optical elements.
It is therefore desirable to monitor any gases, including contaminant gases, in a lithographic apparatus.
Previously, it has been proposed to monitor gases in lithographic apparatus using a known technique called residual gas analysis (RGA). A residual gas monitor is a mass spectrometer which first creates, then analyzes charged particles (ions) using either magnetic or electric fields to separate the ions of different masses. However, it has been found that RGA suffers drawbacks. The residual gas monitor generates heat, and such thermal disturbances in the apparatus may lead to imaging errors. Also, it is a necessary requirement of RGA that the analysis be implemented in situ, that is, in the volume in which the contaminants are originally found. Also, the measurements require a certain amount of time to complete, and are thus quite time consuming.
A further problem with RGA is that it cannot be exposed to high pressures. This limits the dynamical range of the RGA.
Another problem is that because RGA analyzes ions, and not complete molecules, the analysis may be difficult to interpret. A further drawback of the ionization process in RGA is that larger molecules are broken up. This drawback is a particular problem because it is desirable in lithographic apparatus, especially those operating at wavelengths shorter than 157 nm where contamination is a major concern, to identify the precise molecular structure of the contaminants in order to be able to identify their origin, and to be able to gain further insight into the contamination process, which at present is not fully understood. One consequence of having to break up larger molecules in RGA is that extensive calibration of the RGA apparatus is needed. The degree of ionization depends on the molecule and the RGA apparatus setting. To achieve accurate measurements of pressure, these should be calibrated. However, molecules are broken up into ions before they can be identified and measured, thereby adding complexity to the calibration of the RGA apparatus.